1. Technical Field
The present disclosure relates to an arc tube for a discharge lamp unit, and particularly, to a mercury-free arc tube for the discharge lamp unit, which does not contain mercury, used as a light source of a vehicle headlamp.
2. Related Art
In recent years, discharge lamp units have been employed in vehicle headlamps because of advantages of light emitting efficiency and favorable color rendering properties as well as a longer lifetime compared to that of a filament type lamp unit. The discharge lamp unit is configured such that a metal halide arc tube, which is a light source, is supported by a pair of metallic lead supports projected from insulating bases.
In the arc tube, both ends of openings of the quartz glass tube are pinch-sealed, and a closed glass bulb is formed as a discharge portion in the center thereof in a longitudinal direction. At each pinch seal portion, an electrode assembly, which includes a tungsten electrode rod, a Molybdenum foil, and a Molybdenum lead wire integrated in series, is sealed. The front ends of the electrode rods are projected into the closed glass bulb to constitute a pair of electrodes.
The lead wire is led out from the pinch seal portion and is welded to a lead support. The lead support supports the arc tube, and is formed as a current conduction path to the lead wire.
JP-A-9-223481 discloses an arc tube for a discharge lamp unit having a closed glass bulb serving as a discharge portion in which Mercury, NaI, ScI3 and ScBr3 are enclosed together with Xe gas and the amount of enclosed ScBr3 is adjusted to be within the range of about 20 wt % to about 80 wt % relative to the total amount of enclosed ScI3 and ScBr3 (a portion of ScI3 is replaced with ScBr3, which has a larger binding energy (i.e. it is harder for disassociation to occur) than ScI3). With such a configuration, the arc tube for the discharge lamp unit is capable of suppressing a reaction between ScI3 and glass (i.e. devitrification phenomenon whereby glass is eroded) and erosion of the electrodes, thus becoming free from flicker and deterioration in the lumen maintenance factor over a long period of time.
Here, deterioration in lumen maintenance factor is caused by a decrease in light emission of metal iodide enclosed in the closed glass bulb, in particular, scandium iodide (ScI3). The decrease in light emission of scandium iodide is caused because a chemical reaction between scandium iodide and quartz glass (SiO2) occurs and results in loss of scandium iodide. The loss of scandium iodide is caused by the reaction between scandium iodide and quartz glass expressed by chemical formula (1) shown below. As a result, scandium iodide is changed into oxide, and this causes a decrease in Sc vapor pressure, thereby decreasing luminous flux.4ScI3+3SiO2→2Sc2O3+3SiI4  (1)
Further, when the reaction represented by the above formula occurs, not only the loss of scandium iodide occurs, but also erosion (devitrification) of quartz glass occurs. Furthermore, SiI4 created by the above reaction reacts with the tungsten of the electrode rods as expressed by formula (2) below, and this reaction creates SiWn, which has a low melting point. Thus, the electrodes are melted, and the distance between the electrodes increases, thereby rising tube voltage. Further, since iodine (I2) having a high vapor pressure is created and the tube voltage increases, lighting becomes unstable or, worse lighting becomes impossible.SiI4+Wn→SiWn+2I2 (2)
As described above, the main factor in the devitrification is that the inner wall of the quartz glass tube is eroded by the reaction expressed by formula (1) above. At this point, in order to eliminate devitrification, it is necessary to decrease an amount of the reaction between quartz glass and ScI3. Accordingly, bromine (Br), which has a stronger binding energy with Sc than iodine (I), has attracted attention. That is, since a part of ScI3 is replaced with ScBr3, which has a larger binding energy (i.e. it is harder for disassociation to occur) than ScI3, the amount of the reaction between ScI3 and the quartz glass (SiO2) is small, and the progress of the reactions of formulas (1) and (2) above is reduced. From this viewpoint, JP-A-9-223481 was proposed.
On the other hand, JP-A-2005-183165 (see e.g., the fifth and sixth examples), discloses a mercury-free arc tube for a discharge lamp unit in which mercury, which is toxic to the environment, is not enclosed in the closed glass bulb of the arc tube. In the closed glass bulb of the arc tube, not only NaI and ScI3 but also InI and ZnI2 serving as buffer substances substituted for mercury are enclosed together with Xe gas. By adjusting the pressure of sealed Xe gas and the amount of ScI3 enclosed, the mercury-free arc tube has substantially the same initial characteristics (a tube voltage, luminous flux, and initial rise of luminous flux) as the arc tube which contains mercury.
The mercury-free arc tube disclosed in JP-A-2005-183165 has substantially the same initial characteristics as the arc tube which contains mercury. However, the mercury-free arc tube is used under high load conditions such that the pressure of sealed Xe gas is higher and the set tube voltage is lower (current flowing through the electrodes is larger) than that of the arc tube which contains mercury. Hence, the deterioration in lumen maintenance factor, which is caused by the reaction between ScI3 and glass (devitrification phenomenon whereby glass is eroded) or the occurrence of flicker due to erosion of the electrodes, is notable as compared with the arc tube which contains mercury, and thus the lifetime is shortened to that extent.
For this reason, it has been considered that the mercury-free arc tube should sustain lumen maintenance factor and should have a long lifetime by suppressing the reaction between ScI3 and glass (devitrification phenomenon whereby glass is eroded) or the occurrence of flicker due to erosion of the electrodes.
However, FIG. 4 of JP-A-9-223481 shows a change in tube voltage of the arc tube which contains mercury in a case where a percentage of the amount of enclosed ScBr3 is increased. As can be seen from the drawing, during a period up until 2,000 hours have elapsed from the first lighting (0 hours), tube voltages of examples (examples A to D) containing ScBr3 in a range of 5 to 15% are lower by about 10 V than tube voltages of comparative examples 1 and 2 containing ScBr3 at 0% (a case where ScBr3 is not enclosed at all). This is due to the fact that the vapor pressure of ScBr3 is lower than that of ScI3.
In addition, since the arc tube containing mercury is capable of compensating for the deterioration in tube voltage by adjusting the amount of enclosed mercury, the increase in the percentage of ScBr3 to ScI3 is effective for suppressing devitrification and erosion of the electrodes. Meanwhile, in the mercury-free arc tube, which does not contain mercury, the InI and ZnI2 serve as buffer substances in substitution for mercury, but it is not easy to compensate for the significant deterioration in tube voltage corresponding to 10 V. Accordingly, it has been considered that though it is effective to increase “the percentage of ScBr3 to ScI3” for the arc tube containing mercury, this strategy is not applicable to the mercury-free arc tube.
However, for the mercury-free arc tube, the inventors of the present application have conducted an experiment in which the amount of enclosed ScBr3 is increased relative to the total weight of the enclosed substances within the closed glass bulb. As a result, as shown in FIGS. 6 and 7, an unexpected result was obtained in which the luminous flux and the tube voltage at the initial stage of lighting can be increased by slightly increasing the amount (percentage) of enclosed ScBr3.
The reason may be that the vapor pressure within the closed glass bulb is increased by the synergy effect caused by mixing ScI3 with ScBr3. In the case of metal halide arc tube, a NaI—ScI3 based metal produces a complex halide of NaI.ScI3, and thus the vapor pressure within closed glass bulb noticeably increases. Then, by adding ScBr3 thereto, a complex halide of NaI.ScI3.ScBr3 is produced, and it can be expected that the vapor pressure further increases.
That is, it was confirmed that when the amount of ScBr3 enclosed in the closed glass bulb of the mercury-free arc tube is adjusted to be within a given range relative to the total weight of the enclosed substances, it is possible to suppress the reaction between ScI3 and glass (devitrification phenomenon whereby glass is eroded) and thus obtain a desirable lumen maintenance factor and lifetime.